MORPHOLOGY OF THE VERTEBRATE OLFACTORY ORGAN. 331 



(lie Vermuthungzu aussern, ob nicht vielleicht in den Nasen- 

 gruben ein Paar solchen Spalten^ freilicfi in wesentlich 

 veianderter Function und, darum audi Structur, zu erkennen 

 sei."i 



Dr. Dohrn does not enter into any details concerning the 

 suggestion thus made, and does not discuss the question of 

 the olfactory nerve. I am not aware that he has since pub- 

 lished any further observations on this point. The sug- 

 gestion appeared to me, if not untenable, at any rate 

 unprovable, so long as the ordinary account of the develop- 

 ment of the olfactory nerve continued to find acceptance. 



In addition to what has been already said there are, I 

 think, many arguments in favour of this view. Even if we 

 leave out of consideration the buccal and lachrymal clefts, 

 it is well known that in all vertebrates above Amphioxus 

 more or fewer of the visceral clefts undergo modification 

 to a greater or less extent, and that this modification is 

 first felt by the clefts at the two extremities of the series, 

 especially by the anterior ones ; and it is a point worthy 

 of notice that, while the posterior clefts tend simply to 

 disappear, the anterior clefts with their gills are peculiarly 

 prone to persist in a modified form. Thus, the first post- 

 oral or hyomandibular cleft is the only one which remains 

 in Sauropsida and Mammalia. Among Ichthyopsida this 

 cleft is apt to assume the modified form of a spiracle, while 

 its gill loses its respiratory function, and persists as a 

 pseudobranchia. Similarly, the carotid gland of the frog 

 and the choroid gland of Teleosteans are probably other 

 instances of the persistence, in an altered condition, of 

 anterior gills. On the other hand, when reduction is effecte<l 

 in the number of the gills from the posterior end of the 

 series, as in nearly all fishes, the gills and their clefis usually 

 disappear absolutely and completely. 



Again, if the olfactory organ is a gill, we should expect 

 to find the resemblance between the two structures strongest 

 in the most primitive vertebrates. From what has been 

 said already this obviously is the case. Of the various types 

 of vertebrates examined it is in the dogfish alone that we 

 find the intimate relation between the development of the 

 gills and that of the Schneiderian folds. 



Whatever view we may hold as to the ancestry of verte- 

 brates, there can be little doubt that they have not inherited 

 their olfactory organ as such. At any rate, we know as yet 

 of no invertebrates that possess olfactory organs from which 

 the vertebrate olfactory organ could possibly have been 



* 'Ursprung der Wirbelthiere.' Leipzig, 1875, p. 23. 



